1. Field of the Invention
The present invention relates to a synchronous mobile communication system, and more particularly to an apparatus and method for measuring and compensating delay between a main base station and a remote base station interconnected by an optical cable.
2. Description of the Related Art
A cellular mobile communication system divides an entire service area into a plurality of cells and covers the cells using base stations. An overlay cell can be formed between adjacent cells in an actual wireless environment. A user mobile unit can receive signals from at least two base stations associated with the overlay cell. In this case, a handoff procedure must be performed according to a communication control signal exchanged between the base stations so that the cellular mobile communication system can maintain communications for the user mobile unit that moves between the cells. Of course, synchronization must be achieved between the base stations so that a handoff can be supported in a synchronous mobile communication system based upon code division multiple access (CDMA) technology.
In the case of an urban district having a greater population density than a rural district, a cell needs to be divided into a plurality of pico-cells or micro-cells. In this case, the pico-cells or micro-cells are covered by remote base stations having a smaller capacity connected to the main base station. Conventionally, the main base station is designed so that it can include a digital unit for processing a CDMA digital signal and a radio frequency (RF) unit for processing an RF carrier signal. The remote base stations can be connected to the main base station by coaxial cables, wireless connections, optical cables, and so on. Because the mobile communication system supports a high data rate, there is a great deal of interest in using optical cables capable of performing long distance transmissions with low signal loss or degradation.
When the main base station and the remote base stations are separated by the relatively long distance of several to several tens of kilometers, the time period required for transmitting a forward baseband signal from the main base station to the remote base stations is proportional to the distance, and relatively large time delays can be incurred between the main base station and the remote base stations. The relatively large time delays can have different values. The different time delays can cause RF signals from the remote base stations to be asynchronously transmitted. In particular, when a mobile unit communicates with a plurality of remote base stations, the problem of asynchronization between the base and the remote base stations can cause speech quality of the mobile unit to be degraded.
FIG. 1 is a block diagram illustrating the configuration of a mobile communication system employing the typical micro-cell or pico-cell architecture.
In the mobile communication system shown in FIG. 1, three remote base stations (RUs) 21 to 23 are connected to one main base station (MU) 20, and the remote units 21 to 23 convert baseband signals received from the main base station 20 into RF signals to transmit the RF signals to a mobile unit 10. The mobile unit 10 receives three RF signals containing the same data from the remote base stations 21 to 23.
FIG. 2 is a block diagram illustrating components constituting the main base station 20 and one of the remote base stations 21 to 23 shown in FIG. 1. In FIG. 2, one optical communication technique for interconnecting the main base station 20 and the remote base stations 21 to 23 employs synchronous digital hierarchy (SDH).
Referring to FIG. 2, the main base station 20 includes a digital processing unit 20a for processing a digital baseband signal, an SDH processing unit 20b for performing a conversion operation between the digital baseband signal and an SDH frame, and an electric/optical (E/O) interface 20c for performing an interface between the SDH frame and an optical signal. The remote base station 21 includes an E/O interface 21a for performing an interface between the optical signal and the SDH frame, an SDH processing unit 21b for performing a conversion operation between the SDH frame and the digital baseband signal, and a radio frequency (RF) processing unit 21c for performing a conversion operation between the digital baseband signal and an RF signal.
Conventional communication techniques employed by mobile communication systems concentrate on the transmission of data over long distances at a high data rate, without any error, and never consider delay incurred by cables. Synchronous CDMA mobile communication systems are very sensitive to signal phase because CDMA mobile communication systems are based upon the smallest element of data referred to as a chip that goes through digital processes such as user identification, coding, decoding, among others. One chip corresponds to 813.8 nanoseconds (ns) when a chip rate of 1.2288 Mcps is employed. When the remote base stations are not synchronized in the synchronous CDMA system, errors can be incurred in the handoff for the mobile unit moving between areas of the remote base stations, and an operation for detecting a signal from the base station is impossible. As a result, communications cannot be appropriately performed.
Furthermore, as a result of the remote base station and the main base station not being synchronized, various states relating to optical signal transmission cannot be confirmed and a set of processes, such as system state management, error detection, among others, cannot be appropriately performed in the conventional communication system that interconnects the main base station and the remote base stations through optical cables. In addition, when the digital baseband signal is processed, there is a problem in that a propagation delay value caused by the optical cable cannot be simply calculated.